It has long been a goal in the field of object inspection to provide for automatic threat detection. A substantial advance in this art is the system described in the Stein et al U.S. Pat. No. 4,031,545 entitled "Radiant Energy Alarm System". However, the Stein et al patent is limited to responding to a high level of opacity, which is typical of dense material (such as a metal) and thus has limited utility for detecting less opaque objects which include explosives, plastics, narcotics, drugs, etc.
Backscatter imaging has been known at least since the Stein publication, "Flying Spot X-Ray Imaging Systems", which appeared in Vol. 30, No. 7 (Jul. 1972) of Materials Evaluation (pp. 137 et seq).
A further advance was made in respect of imaging such less opaque objects like plastic or explosives or the like by the introduction of American Science and Engineering's "Z" system; see for example "A New X-Ray Scanner to Hinder Hijackers", Fortune, Apr. 28, 1986, page 146 and U.S. Pat. No. 4,799,247.
However, the significant difference between the response of less dense materials (such as plastics, drugs, etc.) to x-ray illumination and the response of highly dense material (such as metal) to the same illuminating radiation renders problematical application of the techniques in U.S. Pat. No. 4,031,545 for detection of threats posed by low Z objects. More particularly, the techniques of U.S. Pat. No. 4,031,545 are based on detecting very dense objects. How those techniques could be used in processing x-ray signals generated by less dense material is not apparent. For example, the presence of metal per se is not necessarily correlated with a degree of threat; in the case of a metal object, it is the shape of the object which is significant. Because of the significant difference in transmissivity between metals and nonmetals, in the presence of a metal in an x-ray image, not only is the presence of the metal readily detectable, but the shape of the metal object is also readily discernable. Thus, the technique described in U.S. Pat. No. 4,031,545 finds utility in merely calling the operator's attention to a region of high density. Since the shape of the high density object is usually readily recognizable, the operator can quite readily determine whether or not the object poses a threat. On the other hand, explosives do not usually carry any particular shape, rather it is the mass or volume of the explosive (as opposed to its shape) which determines the degree of threat. It is not apparent from publications on transmission imaging that mass or volume of low density material can be differentiated.